Torque sensors known in the art rely on a magnetoelastic element attached to a component to sense torsion forces in the component. Deformation in the component caused by applied torque deforms the magnetoelastic element, resulting in a magnetic field that is proportional to the applied torque. A magnetometer disposed near the element detects the magnitude and polarity of the magnetic field, which indicates the magnitude and polarity of the applied torque.
To ensure that the deformation in the magnetoelastic element accurately reflects the torque applied to the shaft, the magnetoelastic element is usually a cylinder tightly coupled to the shaft. Depending on the material and process used to manufacture the magnetoelastic element, however, the applied torque may be so great that it causes irreversible changes in the element, permanently deforming it. For example, elements having a magnetoelastic coating applied to a substrate may be axially compressed before the coating is applied to optimize stresses in the coating. The substrate is ideally kept relatively thin in these types of elements, but minimizing substrate thickness also compromises the substrate's ability to handle larger applied torques.
If permanent deformation of the element occurs due to excessive applied torque, the sensor does not return to zero when the shaft is released from the applied torque. This change in the magnetoelastic element is called “zero-shift” because the zero point of the magnetic field generated by the element shifts due to the permanent deformation.
Current applications often place shafts in environments that allow the shaft to twist more than the magnetoelastic element is capable of handling without permanent deformation.